net: tcp: improve retransmit timeout in 3WHS

In this commit retransmit timeout in 3WHS (syn and syn+ack packet) is
assigned to sysctl_tcp_syntimeout (default: HZ/4 = 250ms).  After
applied this commit we can speed up the process of retransmission for
syn and syn+ack packet.

Signed-off-by: Honghao Liu <tianlan.lhh@alibaba-inc.com>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
net: tcp: add a new tcp retransmission detection mechanism based-on last time out

This commit tries to speed up the process of tcp retransmission.  The 
tcp transmission detection can be categorized into two types.  One is
based-on time out, and another is based-on packet loss.

This new tcp retransmission detection can be as a time out based-on
mechanism.  In tcp_sock structure some new members are defined to save
more information about ack.  Here we save the highest sack seq number.
Then they will be used to detect the packet loss for normal packet and
retransmitted packet.

In tcp_mark_lost_retrans() function if skb->time_seq >= high_acked_seq,
we think that the retransmitted packet has lost.  In tcp_mark_time_lost()
function, we will traverse all packets and detect packets that have lost.

Signed-off-by: Honghao Liu <tianlan.lhh@alibaba-inc.com>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
net: tcp: tcp packet loss detection based-on probe packet

Signed-off-by: Honghao Liu <tianlan.lhh@alibaba-inc.com>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>